Time delayed imaging (TDI) based imaging systems have been developed for detecting defects in wafers used to manufacture integrated circuits prior to printing and etching the desired circuit patterns on the wafers. In this type of imaging system, the wafer is conveyed (scanned) across the field of view of a TDI sensor, such as a Charge-Coupled Device (CCD) camera, which accumulates electric charge in proportion to gathered light reflected by the wafer while it is conveyed across the field of view of the camera. The camera includes an array of light gathering sensor elements, such as CCD elements, organized into a matrix of rows and columns. The elements of a particular row are electrically connected to each other so that the charge caused by the exposure is transferred from element to element along the row to integrate the charge generated by the elements of the row into a pixel reflecting the sum of the charge accumulated by the sensor elements of the row.
The TDI imaging process thereby causes the light gathered by a row of elements to be integrated (summed) over multiple sensor elements, resulting in a pixel that represents the light scattered by a particular region of the imaged wafer as that region is conveyed past multiple sensor elements. The process is then repeated for additional pixels to create a two-dimensional image of the wafer. Integrating the light reflected from a particular region of the wafer over an extended number of sensor elements in the scan direction increases the exposure from the corresponding region of the wafer, which aids in the imaging process by gathering a sufficient amount of light reflected from that region to assist in the detection of small defects, such as point defects in the wafer. This is particularly useful in darkfield imaging, where low levels of light are scattered by the wafer being imaged.
The orientation of the rows of the TDI sensor (referred to as the “sensor direction”) is typically aligned with the direction of travel of the wafer being imaged as the wafer is conveyed across the field of view of the camera (referred to as the “scan direction”). In addition, the rate at which the accumulated charge is transferred along the sensor elements (referred to as the “sample clock rate”) is synchronized with the rate at which the wafer is physically conveyed across the field of view of the sensor (referred to as the “scan rate”). Aligning the sensor direction with the scan direction and synchronizing the TDI sample clock rate with the wafer scan rate produces a digital image in which each pixel of the image corresponds to a particular, static region of the wafer as the image of that region is conveyed across a row of sensor elements.
In a conventional TDI wafer imaging system, the scan direction is carefully aligned with the sensor direction, and the sample clock rate is carefully set to match the physical scan rate of the wafer, so that the light gathered by a row of sensor elements is reflected from a particular, static region of the wafer as the wafer is conveyed across the field of view of the camera. This allows each pixel of the captured image to be precisely correlated with a specific physical region of the wafer under inspection, which allows small defects, such as point defects, to be picked up by the inspection system.
In TDI wafer imaging systems, it is often advantageous to increase the numerical aperture (NA) of the TDI sensor to increase the exposure of the TDI sensor to light scattered by the subject wafer. Increasing the exposure can be particularly useful when attempting to detect small defects, such as point defects, in darkfield optical configurations. It may also be desirable to increase the scan rate of the TDI system, which generally improves the speed of the inspection and thereby improves the overall cost of ownership of the inspection system. However, increasing the scan rate also has the effect of increasing the size of the region of the wafer corresponding to each pixel (pixel size) in the sampled image. Increasing both the NA and the pixel size can result in aliasing when the NA becomes high enough to allow image modulation at frequencies greater than 1/(2× pixel size).
It would be desirable to image point defects at high numerical aperture while also sampling the image at high scan rates, resulting in relatively large pixel sizes, without introducing aliasing in the sampled images. This would enable greater capture of defect energy and higher-speed inspection while minimizing missed defects and false defects. However, the combined effect of increasing the numerical aperture while also increasing the effective pixel size can lead to unacceptable levels of aliasing, which corrupts the captured image and leads to lower defect capture rates and the detection of false defects.
There is, therefore, a continuing need for techniques for avoiding or counteracting the effect of aliasing when increasing the numerical aperture and effective pixel size in TDI imaging systems. More particularly, there is a need for effective anti-aliasing techniques for integrated circuit wafer inspection systems to improve the ability of the systems to capture low-signal defects, such as point defects in darkfield inspection systems, without sacrificing numerical aperture (exposure) or scan rate (pixel size).